Cables such as data cables and telecommunications cables have been used for many years to transmit information. In any cable, a conductor is protected from external influence by one or more sheathing layers which act to shield, protect and insulate the conductor. The conductor is typically a metal wire such as a copper wire which is surrounded by one or more sheathing layers. In their simplest form, a cable may just be copper wire provided with a single layer of insulation. It will be appreciated that much more complex cable designs exist.
In a conventional method, cables are made in an extrusion process in which the conductor is covered with molten polymer which is then cooled in a series of water baths thus creating an insulating layer. The insulation layer is typically a polyethylene polymer.
It is important that the polyethylene used for cable insulation does not have a high crystallization speed (short crystallization halftime), i.e. the polyethylene should not crystallise too quickly. If a polymer has a very fast crystallization rate then the cable insulation layer will shrink on cooling. Shrinkage of the insulation layer is a process which the skilled person wants to avoid. Also, adhesion of the insulation layer to the conductor is poor if crystallisation speed is too rapid, resulting in inferior electrical properties of the produced cable.
It is known in the art that crystallization speed can be slowed by, for example, decreasing polymer density. However, lower density polymers often exhibit poorer mechanical properties. Also, lower density results in higher adsorption of fillers that can be present in the cable, in particular petroleum jelly. Petroleum jelly is routinely used in telephone cable construction to support a group of cables. The use of petroleum jelly was first proposed in 1968 when Bell Telephone Laboratories reported a new cable design where air space in a cable was filled with a dielectric compound consisting of polyethylene and petroleum jelly. In the case of a rupture of the outer sheath, the jelly prevents water ingress, stabilizes electrical transmission, permits use of an economical sheath design, and prevents water from flowing along the cable length.
If the cable insulation material adsorbs the petroleum jelly then cable performance is again compromised. Although the manufacture of traditional copper multipair telephone cables in which petroleum jelly is used is in decline, there remains a need to continue to produce these products in cases where it is more cost effective to extend an existing network rather than install new fibre optic technology. In addition, similar cable filling technology continues to be used for fibre optic cables.
The present inventors targeted a multimodal polyethylene polymer composition for use in the manufacture of the insulation layer in a cable such as a data cable or telecommunications cable. That cable could be a fibre optic cable or a traditional telecommunications or data cable. The cable can comprise fillers such as petroleum jelly.
In this regard, it is known that bimodal polyethylene grades offer superior balance of certain mechanical properties when compared to unimodal polyethylene grades. Multimodal polymers also tend to possess improved processability (corresponding to lower melt pressure in the extruder) due to their broader molecular weight distribution.
Unfortunately, bimodal polyethylenes have faster crystallization speed (shorter crystallization half times) than their unimodal counterparts, resulting in higher shrinkage and inferior adhesion to the conductor in the cable. Also, the resistance to petroleum jelly adsorption is worse in a multimodal polymer making them less than ideal candidates in cables where petroleum jelly or other fillers are present. Whilst therefore, there are benefits to using a multimodal polyethylene in terms of their mechanical and rheological properties, those benefits are outweighed by the negative impact multimodality has on insulation layer shrinkage and adhesion to conductor and possible compatibility issues with fillers such as petroleum jelly.
Therefore there is the need to combine the good mechanical and rheological properties of multimodal polyethylene with slower crystallization speed (longer crystallization half times) and good petroleum jelly adsorption resistance of a unimodal polyethylene. The present inventors have now found that certain multimodal polyethylene copolymers characterised by their high density, relatively high MFR, density split between fractions, and copolymeric structure offer an excellent balance of properties for cable insulation. The polymers offer slow crystallisation speed (long crystallization half times) and good resistance to petroleum jelly. Being multimodal, the polymers also possess excellent mechanical properties, e.g. in terms of their balance of stiffness/stress crack resistance and excellent rheological properties, e.g. in terms of shear thinning index, meaning the polymers are readily processed into cables.
Bimodal polyethylene has been used in the manufacture of cable insulation before. In EP1,159,350 some multimodal polyethylene copolymers are described as supports for use in fibre optic cables. The polymers are however based on polymers with very low MFR, e.g. MFR5 of 0.1 to 2.0 g/10 min. The MFR appears to offer a compromise between processing properties and dimensional stability. Moreover, in the examples, the LMW component in the polymer is a homopolymer. Our higher MFR values are advantageous for extrusion.
EP1,739,110 describes multimodal polymers for use in cable and film applications but these polymers are of low density and therefore lack the mechanical performance of the higher density polymers of the present invention.
The present inventors have therefore devised new polymers with an ideal balance of MFR, density, and density split based on a two copolymer components. They also exhibit advantageous slow crystallization speed (long crystallization half times) as our polymers are based on two copolymer fractions.